Field of the Invention
The present invention relates to a post-processing apparatus configured to perform a post-processing on sheets discharged from an image forming apparatus, and to an image forming system including the post-processing apparatus and the image forming apparatus.
Description of the Related Art
In the prior art, there has been known a post-processing apparatus, which is arranged with an image forming apparatus, e.g., a copying machine, and is configured to perform post-processing such as punching, stapling, and sorting on sheets conveyed from the image forming apparatus. The post-processing apparatus includes a stacking tray configured to allow the sheets, which are delivered after being subjected to post-processing, to be stacked thereon.
In Japanese Patent Application Laid-open No. Hei 2-270762, there is disclosed a post-processing apparatus configured to perform predetermined post-processing on sheets conveyed from an image forming apparatus, then stack those sheets onto a stacking tray, and raise or lower the stacking tray in accordance with the number of the sheets stacked on the stacking tray.
The post-processing apparatus is configured to detect a height of the sheets stacked on the stacking tray, and determine whether or not the detected height has reached a predetermined height, to thereby detect whether or not over-stacking has occurred, that is, detect whether or not the stacking tray has been fully loaded with the sheets.
In Japanese Patent Application Laid-open No. Hei 3-13454, there is disclosed a post-processing apparatus configured to perform predetermined post-processing on sheets conveyed from an image forming apparatus, then stack those sheets onto a stacking tray, and raise or lower the stacking tray in accordance with the number of the sheets stacked on the stacking tray.
In the post-processing apparatus of Japanese Patent Application Laid-open No. Hei 3-13454, the number of the sheets stacked on the stacking tray is counted, and an obtained count value is compared to a predetermined upper limit number of stacked sheets, to thereby determine whether or not over-stacking has occurred.
In Japanese Patent Application Laid-open No. Hei 2-270762, the height of the stacked sheets is detected to determine whether or not over-stacking has occurred. In Japanese Patent Application Laid-open No. Hei 3-13454, the number of the stacked sheets is detected to determine whether or not over-stacking has occurred.
In the method of Japanese Patent Application Laid-open No. Hei 2-270762, when the power of a system is turned off under a state in which over-stacking has occurred, and thereafter the power is turned on, the height of the sheets is detected again by a stacking tray initialization operation performed at the time of turning the power on. As described above, detection of whether or not the over-stacking has occurred is performed again when the power is turned on. Thus, erroneous detection of the over-stacking does not occur.
However, as in Japanese Patent Application Laid-open No. Hei 3-13454, when a determination as to whether or not over-stacking has occurred is made based on the number of stacked sheets or the number of stacked bundles, there is a case where the determination as to whether or not over-stacking has occurred may not be properly made after turning off or on the power of the system, as described below.
The method of determination based on the number of stacked sheets is employed in the cases of delivering, for example, sheet bundles subjected to stapling, or special sheets having a large basis weight. The sheet bundles subjected to stapling become thicker at stapled portions, and hence stapled end portions of the sheets are swelled as compared to other portions. Thus, when the sheet bundles are stacked to the same stack height as the case of stacking normal sheets, the stacked bundles may be unbalanced and dropped.
In view of the above, in the method of determination based on the number of stacked sheets, the upper limit number of stacked sheets is suppressed to a small value. This is for the purpose of enabling the post-processing apparatus to determine that over-stacking has occurred under a state in which a height of stacked sheets is small as compared to the method of determination based on the stack height of the normal sheets. Further, when special sheets having a large basis weight are continuously stacked to an upper limit stack height set for normal sheets, the weight of a pile of those sheets may exceed a load capacity value of the stacking tray. In this case, the torque of the stacking tray may become insufficient. Thus, also with regard to the special sheets having the large basis weight, it is necessary to determine whether or not over-stacking has occurred based on the number of the delivered sheets.
For example, there is a case where it is not determined that over-stacking has occurred based on the height of stacked sheets but is determined that the over-stacking has occurred based on the number of the stacked sheets, and there is a case where the system is shifted to a power-off mode or a sleep mode under this state. In this case, in the stacking tray initialization operation which is performed, for example, upon turning the power on, the height of the stacked sheets is detected again. However, when the system is powered off, or shifted to the sleep mode or a standby mode, information related to the number of stacked sheets is lost. In addition, the height of the sheets has not reached the upper limit height as described above. Thus, there is a risk in that it is not determined that the over-stacking has occurred even under a state in which the determination that the over-stacking has occurred is required to be made.
As a result, even when the power is turned off or on under a state in which the over-stacking has been detected, the sheets are allowed to be delivered, and hence failures in stacking or malfunction of the stacking tray may be induced.